1. Field of the Disclosure
This disclosure resides in the field of the desulfurization of petroleum and petroleum-based fuels.
2. Description of the Related Art
Diesel fuel is one of the three most important fuels, including gasoline, diesel, and jet fuels, which are widely used in transportation. Because diesel engines are inherently more thermally efficient than gasoline engines, it is expected that diesel demand and utilization will increase more in the next few decades in the 21st century. Moreover, diesel fuels are complex mixtures of alkanes, cycloalkanes, and aromatic hydrocarbons with carbon numbers in the range of C9-C28 and with a boiling-range of 150-390° C. Their relative distribution depends on the feedstock, refining process, and blending schemes based on commercial demand. Due to the higher boiling range, two more commonly found sulfur compounds in diesel are alkylbenzothiophenes (BTs) and alkyldibenzothiophenes (DBTs).
Sulfur content in diesel fuel is an environmental concern. Upon combustion, sulfur leads directly to emission of SO2 and sulfate particulate matter (PM), which endangers public health and welfare. Moreover, sulfur in petroleum often poisons catalytic converters, corrodes parts of internal combustion engines, and leads to air pollution. Because the sulfur compounds are poisonous to the shift catalyst in the hydrocarbon conversion process and the electrode catalyst in fuel cell process, the sulfur content of petroleum fuels has to be decreased to less than 0.1 ppm. Therefore, U.S. EPA has issued regulations that require the reduction of sulfur content of gasoline from 300 ppm to 30 ppm, and that of diesel fuel from 500 ppm to 15 ppm. Even more aggressive plans are being discussed or implemented for the future.
The hydrodesulfurization (HDS) or distillate hydrotreating is one of the largest scale chemical processes to remove sulfur from diesel and has been carried out in today's industry. Traditional HDS is a hydro-treatment process that requires hydrogen and a catalyst to break up the sulfur-containing compounds in diesel in order to form hydrogen sulfide.
However, unreacted hydrogen sulfide from the process is harmful, even in very small amounts. Hydrogen sulfide has an extremely high acute toxicity, which has caused many deaths in the workplace and in areas of natural accumulation, and is hazardous to workers. These hazards present health risks in many types of industries, such as the gas, oil, chemical, geothermal energy, mining, drilling, and smelting industries. One of the difficulties with the new regulations is that when hydrodesulfurization is performed under the more stringent conditions needed to achieve the lower sulfur levels, there is an increased risk of hydrogen leaking through walls of the reactor.
Furthermore, recent studies on hydrodesulfurization (HDS) indicate that organic sulfur compounds (OSCs) remaining in diesel fuel at sulfur level lower than 0.1 wt % are alkyl-DBT with alkyl substations at 4- and/or 6-position. Dibenzothiophene (DBT), 4-methyl-dibenzothiophene, and 4,6-dimethyldibenzothiophene are typical refractory sulfur compounds in diesel and other gas oils. These compounds are lower in HDS reactivity and are classified as the most refractory compounds in conventional HDS. Therefore, to reduce those refractory OSCs based on conventional approaches of HDS of diesel fuel, if the sulfur level is reduced from current 0.050 wt % to 0.0015 wt %, the volume of catalyst bed will have to be increased 3.2 times as that for current HDS catalyst bed. If the sulfur is reduced to 0.0001 wt % level, the volume of catalyst bed will have to be increased by about 7 times. It might be difficult to meet the demand by making small improvements in existing HDS technology.
One method for desulfurization of diesel fuels that has shown promise is oxidative desulfurization. Sulfur compounds are known to be slightly more polar than hydrocarbons. However, oxidized sulfur compounds such as sulfones or sulfoxides are substantially more polar than sulfides. More importantly, the oxidation of sulfides to sulfoxides or sulfones is usually much easier and faster than the oxidation of most hydrocarbons. As such, the conversion of slightly polar sulfides to more polar sulfones or sulfoxides allows for the sulfur compounds to be more easily extracted from the fossil fuels into an aqueous phase. The greatest advantages of the oxidative desulfurization (ODS) process are low reaction temperature and pressure and the fact that expensive hydrogen is not used in the process. Another advantage of ODS is that the refractory sulfur compounds in HDS are easily converted by oxidation. The applicability of an oxidative desulfurization scheme depends on the kinetics and selectivity of the oxidation of organic sulfide. Polyoxometalates have long been studied for oxidation reactions, particularly, the polyoxometalate/hydrogen peroxide system for organic substrate oxidations. It has been well documented in previous studies that the tungsten and molybdenum polyoxometalates with a Keggin structure converted to polyoxoperoxo species in the presence of hydrogen peroxide. Even sulfur compounds with low nuleophilicity, such as dibenzothiophene, can be oxidized under mild condition to sulfoxides or sulfones in high yields.
U.S. Pat. No. 6,402,939 describes a technique in which organic sulfur compounds are removed from a fossil (or petroleum-derived) fuel by a process that combines oxidative desulfurization with the use of ultrasound. The oxidative desulfurization is achieved by combining the fossil fuel with a hydroperoxide oxidizing agent in the presence of an aqueous fluid, and the ultrasound is applied to the resulting mixture to increase the reactivity of the species in the mixture. Ultrasound-assisted oxidative desulfurization (UAOD) process operating at ambient temperature and atmospheric pressure permits the selective removal of sulfur compounds from hydrocarbons by a combination process of selective oxidation and solvent extraction or solid adsorption with high yields. However, it was found that bromo by-products were formed by using quaternary ammonium bromides as phase transfer agents (PTA). This is caused by the bromide used as the anion of the quaternary ammonium salt.
Furthermore, ultrasound applied to oxidative desulfurization has accomplished high sulfur removal with a probe type reactor in a batch scale. However, in a batch process, all the reaction components are combined and held under controlled conditions until the desired process endpoint has been reached. Reactions are typically slow, taking hours, and the product is isolated at the end of the process cycle.